Dynamic gas blending

ABSTRACT

The disclosure is directed to automatic flow blending of gas mixtures useful for calibrating gas analyzing instruments of the kind used in measuring atmospheric pollutants and the like. According to the disclosure the component gases reduced in pressure from the storage pressure, are continually mixed in a mixing zone at low pressure and pumped to high pressure for charging high pressure storage vessels. The mixture in the high pressure charging line is continuously monitored with a gas analyzer instrument. Variations from the desired blend are converted into an analog signal which is used to control servodriven valve mechanisms for regulating the flow of gases to the low pressure mixing zone so that a match of the desired blend of gases is obtained.

United States Patent [191 Strain et al.

[451 Dec. 24, 1974 DYNAMIC GAS BLENDING [73] Assignee: ScottEnvironmental Technology,

Inc., Plumsteadville, Pa.

22] Filed: Feb. 22, 1973 21 App]. No.: 334,842

[52] U.S. Cl 137/3, 137/93 [51] Int. Cl. G05d 11/08 [58] Field of Search137/3, 4, 5, 6, 88, 90,

[56] References Cited UNITED STATES PATENTS 1,912,044 5/1933 Schmidt137/6 2,117,819 5/1938 Okada 137/565 X 2,269,674 1/1942 Liddel et al137/4 X 2,660,360 11/1953 Gricar et al 137/565 X 2,713,788 7/1955 Goff137/91 X 2,787,281 4/1957 Word 137/93 2,883,343 4/1959 Favre et al....137/93 X 3,233,987 2/1966 Hepburn 137/7 X Howard 137/88 X Walker 60/285Primary Examiner-Robert G. Nilson Attorney, Agent, or Firm-Synnestvedt &Lechner I [57] ABSTRACT The disclosure is directed to automatic flowblending of gas mixtures useful for calibrating gas analyzinginstruments of the kind used in measuring atmospheric pollutants and thelike, According to the disclosure the component gases reduced inpressure from the storage pressure, are continually mixed in a mixingzone at low pressure and pumped to high pressure for charging highpressure storage vessels. The mixture in the high pressure charging lineis continuously monitored with a gas analyzer instrument. Variationsfrom the desired blend are converted into an analog signal which is usedto control servo-driven valve mechanisms for regulating the flow ofgases to the low pressure mixing zone so that a match of the desiredblend of gases is obtained.

8 Claims, 1 Drawing Figure co co uo H0 DYNAMIC GAS BLENDING BACKGROUNDOF THE INVENTION 1. Field Of The Invention This invention relates to theblending of gases, and especially to the blending of calibration gasesuseful for the calibration of gas analysis instruments and the like.

2. The Problem And The Prior Art Calibration gases have been widely usedin recent years for the calibration of instruments used in the analysisof mixtures of gases. The demand for calibration gases has becomeparticularly active as a result of the need to accurately measure theexhaust emissions from automotive and other internal combustion enginesas well as emissions from industrial sources. With the advent of stateand federal controls governing the emissions of carbon monoxide,hydrocarbons and other exhaust gas components, inspection and testingprocedures have evolved in order to make certain that motor vehiclesmeet the required emissions standards. In fact, it is anticipated thatin the very near future all automobiles manufactured will have to betested as they leave the assembly line to make certain that the emissionof the above pollutants is maintained within prescribed limits. It isalso expected that automobiles now in use will be inspected periodicallyto determine whether or not they are being maintained in a proper stateof tune so that they meet the same or similar high standards.

Present day techniques for gas analysis require the use of an instrumentin which a so-called calibration gas having a composition which isprecisely calibrated is measured to produce an instrument response.Typically this is accomplished by placing the calibration gas in a cellthrough while a beam of infrared light is passed. The response of theinstrument to the presence of the calibration gas, as indicated by theabsorption of infrared energy, is observed and recorded. Thereafter theunknown gas is passed through the cell and similar observations taken.The concentration of the unknown gas may then be determined byobservation of the instrument response and comparison with that obtainedwith the calibration gas.

With increasing need for these and similar instruments, the use ofcalibration gas has increased sharply in recent years. Typically thecalibration gas is supplied to the user in 48 liter cylinders which arepressurized to about 1500-2000 p.s.i. In preparing the gas mixtures,mixing is accomplished by the introduction of the components of the gasblend into the cylinder and pressurizing with the background gas such asnitrogen to the high pressure. The components of the blend behavedifferently at high pressures than they do under atmospheric conditions,not mixing well and exhibiting a tendency to stratify. This isapparently because the mean free paths of the various molecules aresufficiently shortened at the higher pressures so that the individualcomponents do not diffuse readily. Mixing under high pressure is a timeconsuming and expensive process, involving prolonged heating andagitating of the cylinder in order to produce a commercially acceptableproduct. By way of example, mixing times by the prior art approach maybe two hours or more in a typical case.

One drawback with the prior art batch approach described above arisesbecause of the difficulties involved in precisely matching the mixturein one cylinder with that mixed at a previous time within the tolerancesneeded by industry. Since the, measurement of exhaust emission ofteninvolves the use of computerized equipment associated with the analyzer,adjustments in the instrument and or in computer programs associatedwith and controlled by the instrument have to be made each time a newgas cylinder is used. This involves extra time and expense on the partof the user and is a potential source of error in the process of gasanalysis.

Although laboratory techniques for blending gases at atmosphericpressures have been known in the art, so far as we are aware the use ofthese has been limited and there has been no recognition of thedesirability of the production of gas blends in pressurized vessels,controlling the mixing at low pressure by a monitoring of the same blendof gases after they have been elevated in pressure.

Another prior art approach known to applicant involves the weighing inof components using a beam type scale on which the mixing vessel isweighed. Although accurate mixing of gas blends can be accomplishedusing this method, suitable beam type scales are extremely costly andgreat care must be taken to insure that external factors such as dirt,vibrations or the like do not result in errors in the weighing process.

A discussion of prior art methods or producing calibration gases may befound in NON-DISPERSIVE IN- FRA-RED GAS ANALYSIS, D. W. Hill and T.Powell, Plenum Press, New York, New York, 1968.

OBJECTS AND SUMMARY OF THE INVENTION Having in mind the foregoing, animportant object of the invention is the provision of equipment andmethod for generating gas mixtures of specified component concentrationswithin extremely close tolerances.

Another object of the invention is the provision of means and method forthe rapid and efficient mixing of gas blends.

Still another object of the invention is the provision of means andmethod for achieving more complete mixing of gases than has heretoforebeen commercially practicable.

Still another object of the invention is the provision of large volumemixing techniques which make possible more economical production of gasblends on a mass production basis.

Still another object of the invention is the provision of means forsubstantially automatically and accurately blending gases to conform tosamples supplied by a customer.

Still another object of the invention is the provision of apparatuswhich makes possible the mixing of gases for calibration gas purposeswithin customer's storage vessels which are located at the point of enduse. According to this aspect of the invention, the invention permitsthe continuous withdrawal of gas from the storage vessels by thecustomer during the refilling process thereby insuring the customer ofan uninterrupted supply of calibration gas.

A further object of the invention is the provision of means and methodfor gas blending which results in economies for both the blender and theuser.

In summary, the present invention involves a recognition of the factthat gases depart from ideal behavoir as the pressure in the gas mixtureis raised. Complete mixing becomes much more difficult, evidentlybecause the length of the mean free path of the gas molecules variesinversely with pressure, as a result of which the molecules of thecomponents do not intermingle as freely at high pressure as they do atlow pressure. Thus, gases mixed at high pressure may be stratified inthe cylinders in which they are delivered to the end user, and eventhough great care was taken to measure the amounts of the components ofthe blend in the first instance, the customer will not get the desiredblend out of the vessel unless great care is taken during the mixingprocess to properly and thoroughly mix the components.

According to the invention, substantially instantaneous complete mixingof a blend of gases in desired concentrations is achieved by reducingthe blending pressure of the gases to a relatively low pressure, e.g.,preferably up to about two atmospheres and generally not exceeding about50 p.s.i.g.

A gas pump is provided to pressurize the blended gas to the chargingpressure at which the blend is to be stored. Flow control means regulatethe proportions of gases delivered to the point of blending. Controlmechanism for the flow control means includes means for sensing theproportions of the blended gases downstream from the pump and forregulating the flow control means to establish and maintain the mixingof a predetermined blend.

ln carrying out the invention the sensing means establishes theproportions of the components of the blend delivered to the blendingchamber by sensing the composition of a sample previously prepared.Means are provided so a blend already in a storage vessel to be chargedcan be sensed to establish the components of the blend to be deliveredto the storage vessel.

The sensing means comprises an analyzer instrument which continuouslymonitors the high pressure output of the pump thus measuring what isactually being delivered to the customer. Control means responsive tochanges sensed by the analyzer instrument continuously adjust theproportions of the gaseous components of the blend in order to preciselymatch the sample. Various connections are provided so that samples ofnumerous calibration gases may be matched, for instance at a customersplant. Preparation of the apparatus for the production of a differentkind of gas can be made simply and easily by merely evacuating theapparatus and purging so as to insure that no contamination of the newgas blend to be produced takes place.

THE DRAWING:

In the detailed description of the illustrative embodiment, referencewill be made to the accompanying drawing whic shows a diagrammatic formof a gas blending system incorporating the principles of the invention.

DETAILED DESCRIPTION OF THE INVENTION With reference now to the drawing,wherein an illustrative embodiment of the invention is shown, theexplanation which follows will be directed to the blending of twocomponent mixtures of gas. The system described includes source ofsupply for the components of a gas blend comprising a plurality ofpressurized storage vessels which are filled with supplies ofa varietyof gases such as carbon monoxide, carbon dioxide, nitric oxide andvarious hydrocarbons, four such supply vessels being shown in thedrawing. Typically the gaseous mixtures comprise one of the gasesidentified above which is then mixed with an inert gas such as nitrogenor hydrocarbon free air to obtain the desired ratio. Although theinvention is described with reference to binary mixtures of gases,ternary and other multi-component mixtures may be prepared using theinventions herein disclosed.

In the illustrative embodiment, the inert background gas such asnitrogen is supplied via a liquid nitrogen supply vessel shownschematically at 11. A vaporizer 12 provides for the delivery of thenitrogen in gaseous form. The gas supply means also preferably includesmeans for alternatively supplying air from the atmosphere and includesatmospheric probe 13 and an air purifier and pump 14. Preferably, thepump should be hydrocarbon free. In the illustrative embodiment of theinvention a pump having a rated capacity of 5000 cubic feet per hour atabout 50 p.s.i.g. outlet pressure is provided.

A selector valve 15 is provided for selectively delivering air ornitrogen to a mixing chamber to be described subsequently.

Each of the gas supply vessels is provided with a pressure reducingvalve 16 so as to reduce the pressure of the selected component to thedesired mixing pressure, which is a pressure at which the gases to beblended thoroughly mix substantially instantaneously without agitationor heating. Although the mixing pressure may be as high as p.s.i.g.preferred mixing pressures are up to about 2 atmospheres and generallyshould not exceed about 50 p.s.i.g.

In order to select the component to be blended, the gas delivery meansfurther includes selector valves 17 a-d located in branch conduitsl8a-d.

A conduit 19 leads from the junction of branch conduits 18ad. Conduit 19is provided with adjustable flow control means comprising a flow controlvalve 20 which regulates the flow of the components of the gas streamselected by operation of the selector valves 17 a-d. Means for operatingthe flow control valve will be described hereinafter.

Downstream of flow regulator or control valve 20 branch conduit 19 isconnected to a conduit 21 leading from selector valve 15 and thus fromthe source of nitrogen or air. Downstream from the junction of conduits19 and 21 a blending of the selected components takes place in a mixingpassage 22 which may include a chamber 23 wherein the components arethoroughly intermixed at the relatively low pressures. A pressure gauge24 is located in the relatively low pressure mixing zone and as shown inthe drawing is between the mixing chamber 23 and a gas pump orcompressor 25.

According to the illustrative embodiment of the invention, compressor 25is a positive displacement multi-stage pump for compressing the blendedgas (typically under pressures on the order of 2,000 p.s.i.g.). Theoutlet of the compressor is connected to a conduit or charging line 26which leads to a fitting to which a selected storage vessel may beconnected. Four such vessels are shown by the reference characters 27athrough 27d respectively, each being provided with a valve 28a through28d. According to one preferred method of utilization of the invention,the storage vessels 27a through 27d are relatively large vessels, forexample on the order of 50 cu. feet (internal volume). These vessels maybe located on the customers plant in which case the gas is immediatelyavailable for use in the customers instruments via suitable manifoldingor may be located at the gas blending plant in which case it istransferred to smaller cylinders suitable for transportation anddelivery to customers. Whatever the case, each of these vessels has anoutlet line identified as 29a through 29d respectively, each of which isprovided with pressure limiters and with suitable control valving andwith couplings (not shown).

It is important that compressor be oil free and that all parts cominginto contact with the gas stream should be inert with respect to thehydrocarbons or other constituents of the gases to be blended. Variouspositive displacement pumps now on the market are suitable for thepurpose. In addition to the criteria discussed above, in selecting aunit it should be added that the low pressure mixing zone must becompletely isolated from the high pressure zone so that back pressureeffects do not influence the pressure at which mixing takes place.

The invention includes means for sensing and analyzing the gas blend onthe high pressure side of the pump. Although other analyzing means couldbe employed, in the preferred embodiment a conventional nondispersiveinfrared analyzer of known construction is used, the instrument beingidentified by the reference character 30. Instrument 30 is comprised ofa plurality of cells there being one provided for each of the gases thatit is contemplated will be blended.

Instrument 30 is connected to the downstream high pressure side of thecompressor via a line 31, which leads to a selector valve 32. A pressureregulator 33 drops the pressure to a pressure which is about the same asthe pressure at which the blend is being mixed. A flow meter withintegral valve 34 is located downstream of the pressure regulator forregulating the flow.

Selector valve 32 is typically a manually operable 4 port, 3 positionvalve with the common port in communication with pressure regulator 33.One port of valve 32 is in communication with conduit 26 via a line 31.A sample 35 is connected to another inlet port of the selector valve andprovides for delivery of a known sample of gas via another selectorvalve 36 whose inlet ports are connected to various calibrated sampleblends. A line 35a leads to another port of selector valve 32 for supplyof nitrogen or air, used for flushing out and zeroing the instrument.

Also located in the charging line downstream from the junction with line31 is a l80 three port valve 38. Valve 38 is movable from an offposition in which flow through line 26 is blocked to a position in whichflow through the line is permitted or to a venting position in whichflow from the pump is vented to atmosphere via vent line 39. A pressureregulator 39a is located in the vent line and is adjustable so thatduring start-up pump pressure can be limited to the pressure in theparticular vessel 27a-d which is to be filled.

In addition to the facility for furnishing the instrument withcalibration gases via the lines 35 and 35a, a sample may be takendirectly from a selected partly filled storage vessel 27a through 27d orfrom the customers partially used cylinder.

Also located in line 26 is another vent line 40 which is equipped with apressure relief valve 41. This valve is set to open at a suitablepredetermined upper limiting pressure such as 2,500 p.s.i.g.

A line 42 is connected to line 26 downstream from valve 38. Line 42 isprovided with a manually operable ball valve 43, a pressure gauge 44 anda selector valve 45 for selectively directing the gas to a line 46 lineor to the vent line via a line 47. Th vent line 39 as indicated above,is provided with an adjustable back pressure regulator valve 39a. Line46 leads to the charging line 26 on the upstream side of valve 38 andprovides a flow path back to the analyzer instrument when valve 38 isclosed.

A line 48 interconnects line 26 with a vacuum pump 49 for use when it isdesired to purge the system of a particular gas which was previouslyblended.

As indicated above, the means for analyzing the gas, identified by thereference character 30 in the drawing, preferably comprises in a typicalinstallation several non-dispersive infra-red analyzers, each of whichis provided with a sample cell for measuring and comparing theconcentration of one of the components ofa gas to be blended. Four suchanalyzers are shown in the illustrative embodiment, each of which is setup to measure the concentration of one of the four gas compo nents inthe supply vessels 10.

The individual analyzers are selectively connected to control mechanism50, by suitable switch means schematically shown in the drawing andindicated by the reference characters 51a through 51d respectively.Control mechanism 50 forms a part of a servo mechanism device alsoincluding the positioner 52 and a balancing motor 53 which regulates therelative concentrations of the conponents being blended in accordancewith fluctuations sensed by the analyzer so that a precise match withthe preselected blend is maintained.

A suitable strip chart recorder 54 connectible to the analyzers by meansof switches 55a through 55d may also be provided.

In use, two modes of operating practice are followed. When a blendalready in a storage vessel to be charged, such as one of the vessels2711-2741, is to be matched, preferred operating practice is to connectthe selected storage vessel to the charging line 26 with all systemvalves closed. The valve 28ad for the selected storage vessel is openedas is 90 valve 43 and the vessel pressure is then read on gauge 44. 180Valve 45 is turned to the position in which some of the original gas inthe storage vessel is vented out through line 47 and vent line 39 sothat the lines are purged. l80Valve 45 is then turned to the oppositeposition so that gas from the vessel flows through line 26 to line 42,line 46, selector valve 32, pressure limiter 33 and flow regulator 34.The gas flows through the analyzer and the response of the instrument isnoted. Valve 43 is then closed.

The l80 valve 38 is next turned to the position in which compressoroutput is vented through back pressure regulator 39a. The compressor isthen started and the back pressure regulator is adjusted to obtain thesame pressure as that of the storage vessel. The l80 valve 38 is nextrotated to the position in which the mixture is pumped into the storagevessel, the vessel valve 28b being also open. As the vessel is filled,the output of the compressor is continuously monitored on the analyzer30 and the controller 50 adjusts the valve 20 to always maintain themixture at the precise concentration originally in the vessel.

According to the second mode of operating practice, a calibrated sampleis delivered to the instrument via selector valve 36, sample line 35 andselector valve 32. The instrument response is observed and thecontroller thereafter adjusted to deliver the desired concentration. Thesample cell is then flushed and the blending apparatus prepared forfilling a storage vessel as previously described. Selector valve 32 isset in the position in which line 26 is in communication with theanalyzer so that the gas delivered by the compressor is continuouslymonitored.

Since mixing takes place at a pressure at which there is no tendency ofthe components to stratify, the blend is mixed more thoroughly andsubstantially instantaneously. The entire mixing apparatus is quitecompact and the customer is assured of getting a concentration of gaseswhich nearly exactly duplicates the concentration he previously used.Time consuming agitation and heating is unnecessary. Inasmuch as theblend in the high pressure vessels is being continuously measured, itprovides the control parameter for low pressure blending.

The invention described is simple to operate and maintain and inaccordance with one important aspect of it, the entire system may betruck or trailer mounted so that it can be driven to the customersfacility where the customers gas storage vessels are located. Theblending apparatus is connected to the storage vessel, the blend in thevessel analyzed, matched and continuously monitored during the refillingprocess. In this manner the customer is assured of a continual supply ofa gas blend of unvarying concentration and may even continue to draw offgas for use during the blending operation.

We claim:

1. A method of charging a partly filled pressurized storage vessel witha thoroughly mixed blend of gases, the concentrations of which match theblend which is already in the vessel, wherein the charging pressure is apressure at which the gaseous components of the blend do not freelyintermix which method comprises measuring the concentration of thecomponents of the blend in the pressurized vessel, feeding thecomponents of the blend from separate sources to a low pressure mixingzone, mixing the components of the blend in the mixing zone at apressure at which the components of the blend freely intermix,compressing the mixture and filling the vessel with the compressedmixture, monitoring the concentration of the components of thecompressed mixture and adjusting the feed to the low pressure mixingzone in a sense to compensate for departures of the concentrations ofthe mixture from the mixture previously in the vessel to thereby matchthe mixture previously in the vessel.

2. Equipment for charging a high pressure storage vessel with a mixtureof gases, the components of which mixture are present in substantiallyidentical quantities to the mixture remaining in the vessel, comprisinggas delivery means for the supply of the gaseous components of themixture, said supply means including a flow regulating valve forregulating the flow of at least one of the components relative toanother, a mixing conduit for the gases, means for limiting the mixingpressure to a pressure at which mixing of the components takes placesubstantially instantaneously, a high pressure gas compressor having aninlet connected to the mixing conduit and a high pressure outlet, acharging line connected to the high pressure outlet for charging astorage vessel needing replenishment at a relatively high pressure, agas mixture analyzer for analyzing the concentration of the componentsof the mixture of the gases downstream from the compressor, saidanalyzer being in communication with the charging line, means responsiveto departures of the concentrations of the components of the mixturefrom a preselected sample for regulating said flow control valve formatching the mixture blend with the preselected sample, and selectorvalve means operable in one position for connecting one of a pluralityof samples of gases to said analyzer and in another position forconnecting said charging line to said analyzer.

3. Apparatus according to claim 2 wherein said compressor is amulti-stage positive displacement pump.

4. Equipment for charging a high pressure storage vessel with athoroughly mixed blend of gases having a desired predeterminedpercentage composition based on the percentage composition of a samplederived from said storage vessel or from some other previouslycalibrated mixture, wherein the charging pressure is a pressure at whichthe gaseous components of the blend do not freely intermix, comprisinggas delivery means for the supply of the gaseous components of the blendincluding flow regulating valve means for regulating the flow of atleast one of the components relative to another, a mixing conduit formixing the blend of gases, means for limiting the pressure in the mixingconduit to a level at which mixture of the components of the blend takesplace substantially instantaneously, a high pressure gas compressorhaving an inlet connected to the mixing conduit for pressurizing themixed gases to said charging pressure, a charging line connected to theoutlet of said compressor for delivering the pressurized gas to astorage vessel needing replenishment, gas blend analyzer means, valvemeans providing for connection of the analyzer alternatively with thestorage vessel or said previously calibrated reference mixture whereby areference sample derived from either source may be analyzed, said lastnamed valve means further providing for continuous monitoring of thepercentage composition of the pressurized mixture delivered by saidcompressor, and means providing for adjustment of the flow regulatingvalve means in a sense which effects matching of the percentagecomposition of the compressed mixture with the desired predeterminedpercentage composition.

5. A method of charging a high pressure storage vessel with a thoroughlymixed blend of gases having a desired predetermined composition, thecomponents of which are in known concentration, wherein the chargingpressure is a pressure at which the gaseous components of the blend donot freely intermix, which method comprises deriving a reference samplealternatively from the storage vessel or from a previously calibratedmixture, measuring the percentage composition of the reference sample,feeding the components of the blend from separate sources to a lowpressure mixing zone wherein the pressure is maintained at a level atwhich the components of the blend freely intermix, compressing the lowpressure mixture to the charging pressure and adjusting the feed to thelow pressure mixing zone in accordance with variations of the actualpercentage composition of the compressed charging mixture in relation tothe percentage composition of the measured sample composition in a sensewhich effects matching of the composition of the compressed mixture withthe desired predetermined composition.

6. Equipment according to claim wherein said relatively low mixingpressure is a pressure below about 50 p.s.i.g.

7. A method of charging a high pressure storage vessel with a thoroughlymixed blend of gases having a predetermined composition, wherein thecomponents of the blend are present in desired concentrations andwherein the charging pressure is a pressure at which the gaseouscomponents of the blend do not freely intermix, which method comprisesderiving a reference sample having the predetermined compositionalternatively from the storage vessel or from a previously calibratedmixture, measuring the concentrations of the components of the referencesample, feeding the components of the blend from separate sources to alow pressure mixing zone wherein the pressure is maintained at a levelat which the components of the blend freely intermix, compressing thelow pressure mixture to the charging pressure and adjusting the feed tothe low pressure mixing zone in accordance with departures of the actualconcentration of the components of the compressed charging mixture fromthe measured concentration of the components of the sample in a sensewhich effects matching of the composition of the compressed mixture withthe predetermined composition.

8. A method according to claim 7 wherein the mixing zone is maintainedat a pressure below p.s.i.g.

1. A method of charging a partly filled pressurized storage vessel witha thoroughly mixed blend of gases, the concentrations of which match theblend which is already in the vessel, wherein the charging pressure is apressure at which the gaseous components of the blend do not freelyinTermix which method comprises measuring the concentration of thecomponents of the blend in the pressurized vessel, feeding thecomponents of the blend from separate sources to a low pressure mixingzone, mixing the components of the blend in the mixing zone at apressure at which the components of the blend freely intermix,compressing the mixture and filling the vessel with the compressedmixture, monitoring the concentration of the components of thecompressed mixture and adjusting the feed to the low pressure mixingzone in a sense to compensate for departures of the concentrations ofthe mixture from the mixture previously in the vessel to thereby matchthe mixture previously in the vessel.
 2. Equipment for charging a highpressure storage vessel with a mixture of gases, the components of whichmixture are present in substantially identical quantities to the mixtureremaining in the vessel, comprising gas delivery means for the supply ofthe gaseous components of the mixture, said supply means including aflow regulating valve for regulating the flow of at least one of thecomponents relative to another, a mixing conduit for the gases, meansfor limiting the mixing pressure to a pressure at which mixing of thecomponents takes place substantially instantaneously, a high pressuregas compressor having an inlet connected to the mixing conduit and ahigh pressure outlet, a charging line connected to the high pressureoutlet for charging a storage vessel needing replenishment at arelatively high pressure, a gas mixture analyzer for analyzing theconcentration of the components of the mixture of the gases downstreamfrom the compressor, said analyzer being in communication with thecharging line, means responsive to departures of the concentrations ofthe components of the mixture from a preselected sample for regulatingsaid flow control valve for matching the mixture blend with thepreselected sample, and selector valve means operable in one positionfor connecting one of a plurality of samples of gases to said analyzerand in another position for connecting said charging line to saidanalyzer.
 3. Apparatus according to claim 2 wherein said compressor is amulti-stage positive displacement pump.
 4. Equipment for charging a highpressure storage vessel with a thoroughly mixed blend of gases having adesired predetermined percentage composition based on the percentagecomposition of a sample derived from said storage vessel or from someother previously calibrated mixture, wherein the charging pressure is apressure at which the gaseous components of the blend do not freelyintermix, comprising gas delivery means for the supply of the gaseouscomponents of the blend including flow regulating valve means forregulating the flow of at least one of the components relative toanother, a mixing conduit for mixing the blend of gases, means forlimiting the pressure in the mixing conduit to a level at which mixtureof the components of the blend takes place substantiallyinstantaneously, a high pressure gas compressor having an inletconnected to the mixing conduit for pressurizing the mixed gases to saidcharging pressure, a charging line connected to the outlet of saidcompressor for delivering the pressurized gas to a storage vesselneeding replenishment, gas blend analyzer means, valve means providingfor connection of the analyzer alternatively with the storage vessel orsaid previously calibrated reference mixture whereby a reference samplederived from either source may be analyzed, said last named valve meansfurther providing for continuous monitoring of the percentagecomposition of the pressurized mixture delivered by said compressor, andmeans providing for adjustment of the flow regulating valve means in asense which effects matching of the percentage composition of thecompressed mixture with the desired predetermined percentagecomposition.
 5. A method of charging a high pressure storage vessel witha thoroughly mixed blend of gases having a desired predetermiNedcomposition, the components of which are in known concentration, whereinthe charging pressure is a pressure at which the gaseous components ofthe blend do not freely intermix, which method comprises deriving areference sample alternatively from the storage vessel or from apreviously calibrated mixture, measuring the percentage composition ofthe reference sample, feeding the components of the blend from separatesources to a low pressure mixing zone wherein the pressure is maintainedat a level at which the components of the blend freely intermix,compressing the low pressure mixture to the charging pressure andadjusting the feed to the low pressure mixing zone in accordance withvariations of the actual percentage composition of the compressedcharging mixture in relation to the percentage composition of themeasured sample composition in a sense which effects matching of thecomposition of the compressed mixture with the desired predeterminedcomposition.
 6. Equipment according to claim 5 wherein said relativelylow mixing pressure is a pressure below about 50 p.s.i.g.
 7. A method ofcharging a high pressure storage vessel with a thoroughly mixed blend ofgases having a predetermined composition, wherein the components of theblend are present in desired concentrations and wherein the chargingpressure is a pressure at which the gaseous components of the blend donot freely intermix, which method comprises deriving a reference samplehaving the predetermined composition alternatively from the storagevessel or from a previously calibrated mixture, measuring theconcentrations of the components of the reference sample, feeding thecomponents of the blend from separate sources to a low pressure mixingzone wherein the pressure is maintained at a level at which thecomponents of the blend freely intermix, compressing the low pressuremixture to the charging pressure and adjusting the feed to the lowpressure mixing zone in accordance with departures of the actualconcentration of the components of the compressed charging mixture fromthe measured concentration of the components of the sample in a sensewhich effects matching of the composition of the compressed mixture withthe predetermined composition.
 8. A method according to claim 7 whereinthe mixing zone is maintained at a pressure below 50 p.s.i.g.